Fig. 21.
Fig. 21.
That the action of the rudder, when forced over till it is at an angle with the keel, is to act as a drag on that side and so deflect the boat’s course, is plain enough. But it is not so obvious a fact that this action of the rudder in turning the boat is not to turn her bow round through the water, but to push the stern sideways while the bow is almost at a standstill. For the centre of rotation of a boat, that is, the imaginary pivot on which she turns, is always well forward.
In ([Fig. 22], A is the centre of rotation. So when the rudder is put over to the right, the boat will revolve on the pivot A till she is in the position indicated by the dotted lines. It will be observed that the stern has moved about twice as far as the bow. The further forward the centre of rotation the greater will this disproportion be.
It is very important to remember this effect when sailing very near any object such as a buoy, for while steering so as to turn the boat’s head away from the object and avoid it, the stern is made to approach the object, and the very action that seems calculated to prevent a collision may become the cause of one.
Fig. 22.
Having shown what are the relations of the sails, hull, and rudder of a boat to the wind and water, and explained how a vessel requires either ballast or beam to prevent the wind from capsizing her, and needs draught to increase her lateral resistance and prevent her from being blown to leeward, it remains to add that the longitudinal resistance to the water must be diminished as much as possible, so that the boat can slip easily through the water and travel with speed.